The present invention relates generally to four-wheel-drive systems of vehicles and more particularly to a transfer device for changing distribution of driving power between two-wheel drive and four-wheel-drive in a part-time (or selective) type of four-wheel-drive vehicles. More specifically, the present invention relates to a transfer device of the above stated character including a system for engaging and disengaging a transfer clutch (or jaw clutch) by using a hydraulic actuator.
Relative to the part-time type four-wheel-drive vehicles of this kind, the present inventor has previously made a number of proposals. These proposals were based on the two-wheel drive systems of front engine front drive (FF), rear engine rear drive (RR), and front engine rear drive (FR). In each case, driving power is transmitted to the front or the rear wheels through engagement of a transfer clutch of a transfer device at the time of four-wheel-drive. The transfer clutch is generally actuated by either of two methods. One is a manual method in which the driver actuates the transfer clutch directly by manipulating a lever. The other is a power-actuating method using a motive power source.
Power-actuating transfer devices have been known as disclosed, for example, in Japanese Patent Laid-Open Publication (Kokai) No. 61-21829. In the case of this device, a rail on the side of a sleeve of the transfer clutch of the transfer device is coupled to an actuator of diaphragm type. The negative pressure in the intake pipe is utilized to operate a diaphragm actuator. Thus the rail and the sleeve are moved forwardly or rearwardly. As a result, the transfer clutch is engaged or disengaged.
In this transfer device of the prior art, the actuator of the transfer clutch is of a diaphragm type utilizing the negative pressure of the engine intake pipe. For this reason, it possesses the advantage of being applicable also to a manual type transmission. However, the actuator capacity is large. For this reason the cost becomes high. Furthermore, the place of mounting the actuator is restricted. In addition, the negative pressure of the engine intake pipe is utilized. As a consequence, there arise problems such as failing to change over when the negative pressure is low at the time of high load.